When casting strip according to the twin roll casting process, the oppositely-rotating casting rolls of the caster delimit the longitudinal sides of the casting gap. On its narrow sides, however, the casting gap is sealed by a side plate in each case.
The side plates are usually comprised of an insert and a support plate, which bears the insert. The insert in this case is usually made from refractory material and is formed so that it partly covers the front sides, associated with it, of the casting rolls and completely covers the narrow side, to be delimited by it, of the casting gap.
The relative movements, necessary in the casting process, between the side plates and the front sides, of the casting rolls as well as the contact with the molten metal moving through the casting gap inevitably lead to wear of the side plate due to abrasive wear. This is particularly true if the contact between the side plates and the molten metal or the casting rolls is realised by means of an insert made from refractory material.
In order that, in the case of side plates equipped with such inserts, it is ensured that the side plates perfectly fulfil their sealing function also at the start of the casting process, it is necessary to grind in the inserts before the casting process begins. For this purpose the respective side plate is adjusted while the casting rolls are rotating until its inserts lie with the required contact pressure against the respective associated front sides of the casting rolls. Thereupon abrasive wear of the insert material occurs in the region of the contact surfaces.
This condition is maintained until the casting rolls are ground in to a certain required depth into the refractory material of the insert and the insert lies closely fitting against the front sides of the casting roll. The inserts of the side plates now have one section, the so-called “positive insert”, projecting into the casting gap, and two sections, the so-called “insert grinding surfaces”, adjacent thereto, springing back in relation to the positive insert, with which sections they lie against the front sides of the casting rolls. Here the positive insert with its lateral edge surface covers a narrow margin, adjacent to the respective casting roll front side, of the peripheral face of the casting rolls in each case.
Corresponding to the cross-sectional shape, determined by the casting roll form, of the casting gap, the width of the positive insert in the casting direction reduces continuously following the peripheral radius of the casting rolls, while the insert grinding surfaces are usually dimensioned so that their width remains uniform over the entire arc of contact with the casting rolls.
Because the side plates, possibly by means of their respective insert, are in direct frictional contact with the cooled casting rolls during the casting process, heat is constantly removed from them. Therefore, during the casting process, the temperature of the side plates is usually lower than the temperature of the molten mass coming into contact with them. Consequently heat is also removed from the molten mass when it touches the side plates. The heat loss in this case can be so great that molten mass solidifies on the respective positive insert. In the case of side plates equipped with an insert made from refractory material, this heat dissipation takes place particularly in the region of the positive insert.
Such solidifications, in technical parlance also known as “parasitic solidifications”, form in practice particularly in the lower third, seen in the casting direction, of the tapering part of the positive insert. Furthermore parasitic solidifications can also form in the region of the so-called triple points at which the respective side plate insert, the respective casting roll and the bath surface of the molten metal flowing through the casting gap converge. The places where the solidifications form are usually equally distributed on both side plates.
Whether parasitic solidifications actually form depends on various factors. Thus the quality of the molten mass that is to be cast (melting range, solidification enthalpy, nucleating agents etc.) is paramount in determining the tendency to form parasitic solidifications. The formation of parasitic solidifications is also affected by the free space available above the bath surface level of the casting gap up to the cover of the casting gap and the vertical lowering means and mould levels contingent thereon.
Moreover the temperature distribution in the side plates or their inserts has substantial influence on the formation of parasitic solidifications. This depends firstly on the axial and vertical speeds, at which the side plate is moved. Secondly the temperature distribution depends on the material of the insert and the currents which can arise near the inserts and are determined by the distance of the immersion nozzle, through which the molten mass is fed into the casting gap, from the insert as well as the kinetic energy of the molten mass entering the casting gap.
Parasitic solidifications can cause strip defects, such as thickness discontinuities (deviation from defined tolerances), insufficient through-solidification (bulging aspects) and material tears (strip edges). In extreme cases they may require the casting process to be aborted.
A further manifestation of the wear on the inserts of the side plates results as a consequence of strip shells forming on the cooled casting rolls during the strip casting process. Increased material wear occurs due to the relatively high strength of these strip shells along the casting roll arc of contact. This material wear leads to the formation of a gap between the particular casting roll and the insert associated with it, whose width (growth and swarming of the strip shell) and depth (erosion=casting material strength and/or corrosion=casting material analytics) increase starting from the level of the molten mass cast into the casting gap towards the narrowest roll distance.
At the corresponding size, the gap formed in the insert by the removal of material promotes the formation of so-called “T-edges”, which are flanked by molten mass, so that the insert and the respective solidifying strip shell can become positively clamped together. As a consequence of the contraction, accompanying the solidification of the molten mass, of the strip width, tensions can then occur in the strip, which tensions become so great that the strip tears in the longitudinal direction. Such longitudinal tearing may also require the casting to be aborted.
It is known that wear of the insert due to contact with solidifying molten mass can be reduced by moving the insert during the casting process in a precisely pre-determined way, relative to the molten mass that is being cast. For this purpose the side plates are oscillated rotationally about a centrally arranged axis. With this rotational oscillation it is equally possible to counteract solidifications in the lower third of the insert and to counteract the wear increasing downwards. Examples of this procedure are described in Japanese Patents JP 03-174954, JP 05-237603 or U.S. Pat. No. 5,188,166.
Common to the known possibilities of minimising wear by oscillation, however, is the disadvantage that in the case of rotational oscillation the fulcrum of the oscillation cannot be shifted or, in the case of horizontal oscillation of this movement, no rotational movement can be superimposed. A shift of the fulcrum may, however, then be necessary in the case of rotational oscillation, for example if the level of the surface of the bath is varied. Likewise a shift of the fulcrum may be necessary in the horizontal direction if higher-strength or more chemically aggressive materials, materials with different solidification behaviour, materials with varying viscosities and not least different casting thicknesses are to be produced.
A further disadvantage of the rotation about a certain central axis, used in the state of the art described above, is that lowering of the inserts is only possible to a limited extent, since here the fulcrum is shifted in or against the casting direction. Such lowering however is expressly recommended for example in WO 04/000487 (EP 1 515 813 B1) for minimising wear or in U.S. Pat. No. 6,296,046 for aligning the insert lower edge with the so-called “kissing point”, at which the strip shells solidified on the peripheral faces of the casting rolls are pressed against one another.
In order to ensure mobility of the side plates proposed in each case, so-called “supporting structures” are employed in practice. These usually comprise a support frame. This supports the adjustment devices needed for adjusting the position of the side plate in the way proposed in each case. Here the support frame can be held in guides, in which it is adjustably mounted in a horizontal and/or vertical direction and can be supported in relation to the firm substrate, on which the particular caster stands (WO 04/000487).